1. Field of the Invention
The present invention relates to an amplifier circuit which is connected to a sensor and capable of variably setting an amplification property and a control method thereof.
2. Description of the Related Art
With the latest advancements in electronics technology, there have been increasingly used sensors in various scenes, which sensors output a detection signal in accordance with a change in physical quantity such as acceleration and angular speed. As sensors are more and more miniaturized and have higher sensitivity, the individual difference in sensor sensitivity tends to increase owing to manufacture variation. This leads to such a problematic situation that when a plurality of sensors detect the same change in physical quantity, there are variations in the magnitude of detection signals output from them.
To solve such a problem, a sensor is often modularized together with an amplifier circuit for amplifying the detection signal of the sensor. In this module, the amplifier circuit consists of a circuit capable of variably setting an amplification property. Upon modulation of the sensor and the amplifier circuit, an amplification property is adjusted according to the sensitivity of the sensor. More concretely, when the sensor is brought into its reference state, an amplification property of the amplifier circuit is adjusted such that the output of the amplifier circuit has a specified value. The adjusted amplification property is stored in a memory circuit such as a flash memory or ROM incorporated in the amplifier circuit, and thereafter, the detection signal of the sensor is amplified by use of the amplification property. Therefore, the module composed of such a sensor and amplifier circuit has the advantage that if the amplifier circuits of different modules detect the same change in physical quantity, they will output signals of the same magnitude so that the above-described problem of the individual difference in sensor sensitivity can be overcome. However, there still remains another problem that even though sensor sensitivity is adjusted for every module, the magnitude of signals output from the plurality of modules upon inputting of the same change varies, if the amplification property of the amplifier circuits varies owing to change with time or temperature change.
A system for adjusting an amplification property according to changes in the environment is disclosed in Japanese unexamined patent publication No. H6(1994)-85940.
This system has an amplifier circuit for amplifying an audio signal from an interphone; a reference sound generator installed in the vicinity of the interphone, for generating a reference sound of a constant level; and an amplifier circuit for amplifying an audio signal. This system further includes an A/D converter circuit for converting an audio signal into digital data; a gain adjustment look-up table in which the output data pieces of the A/D converter circuit are shown in conjunction with their corresponding gain adjustment data; and a gain adjustment circuit for adjusting the gain of the amplifier circuit.
In this system, the adjustment of an amplification property is made in the following way: First, a reference sound generated by the reference sound generator is taken in by the interphone as an audio signal which is in turn amplified by the amplifier circuit to be output as an audio signal. Then, this audio signal is converted into digital data by the A/D converter circuit and the gain adjustment data corresponding to the digital data is obtained from the gain adjustment look-up table. Then, a gain adjustment to the amplifier circuit is made based on the gain adjustment data. In the system of Japanese unexamined patent publication No. H6(1994)-85940, even if there occurs a change in the condition (e.g., the resistance of the line) of the line for connecting the reference sound generator to the interphone, the influence of the change of the line condition can be eliminated because gain is adjusted through the line based on the audio signal corresponding to the reference sound.
The technique for gain adjustment of an amplifier circuit disclosed in Japanese unexamined patent publication No. H6(1994)-85940 can be utilized, by replacing the interphone incorporated in the system of Japanese unexamined patent publication No. H6(1994)-85940 with the sensor of the above-described module constituted by a sensor and an amplifier circuit.
More specifically, if the module composed of a sensor and an amplifier circuit is provided with a sensor reference input device for inputting a reference physical quantity to a sensor which input device is equivalent to the reference sound generator of Japanese unexamined patent publication No. H6(1994)-85940, an amplification property of the amplifier circuit can be adjusted based on an output signal obtained upon inputting of the reference physical quantity to the sensor, similarly to the system disclosed in Japanese unexamined patent publication No. H6(1994)-85940.